You are correct in theory.  The real world is different though. 

In practice most C compilers are lax about checking for this and don't
produce more than a single read/write data segment.  For instance, I just
tried that program on Solaris using gcc and it worked exactly as I expected.
Not even a warning. Most good C++ compilers generate a warning at
initialization warning that a non-const pointer was initialized pointing to
a const object but then C++ is more strict and you can circumvent that check
with a cast anyhow.

This is a common (albeit little known) problem in most languages and its
simply up to the programmer to not modify string literals.

Todd Blanchard

> -----Original Message-----
> From:	Adam P. Jenkins [SMTP:ajenkins at javanet.com]
> Sent:	Wednesday, December 09, 1998 5:31 PM
> To:	squeak at cs.uiuc.edu
> Subject:	RE: string sharing (possible bug?)
> 
> It is illegal to modify string literals in C or C++.  That's why the C
> compiler is free to share string literals.  Many compilers place string
> literals in the read-only section of the program image, so your foo()
> function would get a segmentation fault at runtime.  Most modern C
> compilers
> would also warn you at compile time if you tried to modify a string
> literal
> directly.  Obviously if you modify it through a char* the compiler may not
> be able to tell that it points to a string literal.
> 
> I've used a lot of different programming languages, and Smalltalk is the
> only language I've seen that shares string literals *and* allows them to
> be
> modified.  That's why I thought it was a bug when I discovered this.
> 
> > FWIW, C compilers can share string literals too and the exact same
> problem
> > can result.
> >
> > void foo()
> > {
> >    char* b = "abc";
> >    printf("%s\n",b);
> >    b[1] = 'd';
> > }
> >
> > void main()
> > {
> >    foo(); // prints abc
> >    foo(); // prints abd
> > }
> >
> > Java also shares equivalent strings but lacks this problem because
> strings
> > are always immutable (mods produce new strings).
> 
> Python, and I believe ML, also have immutable strings.
> 
> Adam